1. Field of the Invention
This invention relates to a developing apparatus, an apparatus unit and an image forming method which are used when a latent image formed on an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member is developed to render it visible by the use of a developer carried and transported on a developer carrying member.
2. Related Background Art
Developing apparatus conventionally used when electrostatic latent images formed on, e.g., a photosensitive drum serving as a latent image bearing member are rendered visible by the use of a toner which is a one-component type developer may include those of the following system: Positive or negative electric charges are imparted to toner particles by the mutual friction between toner particles, the friction between a developing sleeve as a developer carrying member and the toner particles and the friction between a member for regulating toner coat quantity on the developing sleeve (developer layer-thickness regulating member) and the toner particles. The toner thus charged is coated very thin on the developing sleeve and then transported to a developing zone at which the photosensitive drum and the developing sleeve face each other. In the developing zone, the toner is caused to fly and adhere to the electrostatic latent image formed on the surface of the photosensitive drum, to make the electrostatic latent image into a visible image.
As the developer carrying member used in such a conventional developing system, a member is used which is produced by molding, e.g., a metal, an alloy or a metal compound into a cylinder and treating its surface by electrolysis, blasting or filing so as to have a stated surface roughness.
When, however, such a developer carrying member is used, in the developer layer formed by the developer layer-thickness regulating member on the developer carrying member surface, the developer which is present in the developer layer on the developer carrying member surface and in the vicinity thereof comes to have a very high electric charge, so that it is strongly attracted to the developer carrying member surface by the action of mirror force. This makes the toner particles have no opportunity of their friction with the developer carrying member, and hence the developer may come to have no preferable electric charges. Thus, if images are formed under such a condition, no satisfactory development and transfer can be carried out, resulting in images with much uneven image density and many black spots around line images.
In recent years, toners are sought to have a smaller particle diameter so that the developer can be fixed at a lower temperature for the purpose of energy saving and highly minute images can be formed. Then, in the case when the above conventional developer carrying member is used in a machine of the type making use of a toner having such a small particle diameter, it has been difficult, as explained below, to well realize the fixing of developer at a low temperature and the formation of highly minute images. For example, there is a tendency that, for the purpose of low-temperature fixing of a developer, glass transition temperature Tg of the developer is set a little lower or a low-melting substance such as wax is added in toner particles in a little larger quantity. However, the developer having such toner particles may be affected by temperature rise of the body of an apparatus such as an electrophotographic apparatus, and tends to melt-adhere to the surface of the developer carrying member to cause a decrease in image density, white lines and blotchy images (caused by an uneven coat of the coat layer on the developer carrying member) in some cases.
Japanese Patent Applications Laid-open No. 1-112253 and No. 2-284158 disclose a proposal of using toners having small particle diameters so that image quality can be made higher and images can be made more highly minute. Such toners having small particle diameters have a larger surface area per unit weight, and hence tend to have a larger electric charge on the surface, where the toner may stick or adhere to the surface of the developer carrying member because of the phenomenon of what is called "charge-up", so that the developer fed afresh onto the developer carrying member can be charged with difficulty and the developer tends to have a non-uniform charge quantity. This tends to cause sleeve ghost on images, and the resultant images tend to be formed as non-uniform images such as images with lines and fogged images in solid black images and halftone images.
In order to prevent occurrence of such a developer having excessive electric charges and prevent strong adhesion of the developer, in Japanese Patent Applications Laid-open No. 1-277256 and No. 3-36570 a method is proposed in which a coat layer of a resin with a conductive material such as carbon black or graphite powder or a solid lubricant dispersed therein is formed on the surface of the developer carrying member. However, with regard to the resin that forms such a coat layer, only a few resins have properties which can impart positive triboelectric charges to toners, in particular, positively chargeable developers. Hence, although excessive charging of the toner can be prevented, it is difficult to make the toner retain the charge quantity at a higher level.
In Japanese Patent Application Laid-open No. 8-179617, a method is also proposed in which a coat layer of a resin to which particles chargeable to a polarity opposite to that of a toner have been added is formed on the above developer carrying member. In this method, however, the particles are further added to the resin incorporated with carbon or graphite, and hence the coat layer may have a low strength. In such an instance, it is difficult to provide images with good quality over a long period of time.
Japanese Patent Application Laid-open No. 5-232793 discloses a developing apparatus comprising a developer carrying member having as a surface layer a resin coat layer which contains at least resin, graphite and carbon black and is so formed that a charge control agent is present at the surface and the vicinity thereof, in order to control the chargeability to toner. As the charge control agent, exemplified are various charge control agents including quaternary ammonium salts. As the resin, exemplified are various resins including phenol resins, polyamide resins and polyurethane resins.
However, this prior art shows an Example in which specifically development is carried out using a negatively chargeable toner on a resin coat layer employing a phenol resin as the resin and nigrosine as the charge control agent, and has no disclosure at all as to positively chargeable toners and how positive triboelectric charges can be imparted preferably when in what combination the resin and the charge control agent are used.
For the purpose of imparting a high positive charge to toner, Japanese Patent Application Laid-open No. 7-114270 discloses a charge-providing member for developing electrostatic latent images which has at least at part of the surface a quaternary ammonium salt compound having a specific structure. It discloses that the above compound is used together with optionally a binder resin or molding resin component to form a coat layer. As the binder resin or molding resin component, used are styrene resins, styrene-acrylic copolymer resins, polyester resins, epoxy resins and mixed resins of any of these, or any of these having an amino group on the alkyl side chain. In its Examples, styrene-acrylate resin is used.
According to studies made by the present inventors, however, in the case of a developer carrying member on which such a coat layer is formed using the quaternary ammonium salt compound and the styrene-acrylate resin in combination, the quaternary ammonium salt compound is present only in the state it is dispersed merely in the styrene-acrylate resin. Thus, as shown in Comparative Examples given later in the part of Examples, the charging property of this coat layer is positive chargeability, and hence the ability to impart positive triboelectric charges to positively chargeable toners is also not sufficient.